The warming of the Earth has cooled the yields of corn and wheat in much of the world, a new study finds.

Although agricultural advances have pushed global production of staple crops skyward, hotter temperatures in Russia, China, Mexico and elsewhere have stunted that growth and contributed to the long-term rise in food prices, says the analysis published Thursday in the journal Science.

“This is tens of billions of dollars a year in lost [agricultural] productivity because of warming,” said David Lobell, an Earth scientist at Stanford University and an author on the report.

Three decades of global warming crimped worldwide yields of corn by about 5.5 percent and wheat by about 3.8 percent compared with what would have been produced had world temperatures remained stable, the report says.

A burgeoning global population also needs more crops — and more grain-fed beef — which contributes to rising food prices much more than climate change, Lobell said. This week, the United Nations also projected that the global population will hit 7 billion in October and 10.6 billion by 2050. Such a huge increase will continue to push food prices higher.

For now, the bread basket of America bucked the trend, as agricultural regions of the United States have not warmed much during their growing seasons since 1980. Climate scientists debate the reasons, with some pointing to particulate pollution over the middle of the United States as a possible cooling counterbalance.

This climate hit adds about 6 percent to the cost of wheat and corn, staples whose prices have skyrocketed in recent years. Although global warming is “a small part of the overall story of why prices are going up,” Lobell said, “it’s not negligible.”

Global corn prices doubled between April 2010 and April 2011, the United Nation’s Food and Agriculture Organization reported Thursday. Wheat prices are up 60 to 80 percent depending on the strain, said Abdolreza Abbassian, an FAO analyst.

Diversion of corn for ethanol also adds to its rising cost, said Wolfram Schlenker, an economist at Columbia University who contributed to the Science report. “We need increasing yields to continue feeding the world,” he said.

Lobell and colleagues compiled temperature, rainfall and crop production data for growing regions around the world from 1980 through 2008. They found a striking increase in temperatures in many growing regions during the growing season.

The team then compiled how much corn, wheat, rice and soybeans were grown in every country in 2008. They compared those figures with projections of how much of each crop could have been grown had global temperatures not risen since 1980.

Production of rice, which thrives in warm temperatures, and soybeans, which are largely grown in regions that did not experience much warming, were not affected by climate change.

The study also found that increased atmospheric carbon dioxide — which many crops absorb for growth — boosted production of rice, wheat and soybeans. But for wheat, this boost was more than drowned out by the negative impact of higher temperatures.

Wheat production in Russia took the biggest hit — with yields about 15 percent lower than they might have been — as that country’s arable region has warmed substantially since 1980.

Ken Cassman, a professor of systems agronomy at the University of Nebraska at Lincoln, challenged the study. “It’s not clear how well these analyses are capturing how well farmers can respond, and have been responding, to changing temperatures,” he said.

Lobell said the study did not try to assess any adaptations adopted by farmers by, for example, adjusting planting schedules or moving to cooler locales. But it’s a question he and his colleagues are pursuing.

The study should motivate governments and seed companies to develop heat- and drought-resistant crops, Schlenker said. “In the last 60 years, there have been close to no advances in making crops less sensitive to extreme heat,” he said.

Likewise, farmers around the world will need to adapt to a rapidly changing climate, said Earth Policy Institute President Lester Brown.

And although U.S. agriculture might not be feeling the worst effects of climate change, Brown said, changing precipitation patterns are affecting the nation’s planting schedule. Typically, 40 percent of the nation’s corn crop is planted by May 1; this year only 13 percent is in the ground because of heavy rains in the Midwest and northern plain states.

“We are way behind, and we’re not going to catch up quickly,” Brown said, adding that if farmers can’t plant much of the corn crop by May 10, it could have a significant impact on the year’s overall yield.

The nation’s consumers might end up paying more for food as other countries experience shortages.

“There’s no question . . . the price effects are going to be felt much more broadly,” said David Waskow, climate change program director for Oxfam America.

North American farmers will initially see greater yields as the climate warms, according to the United Nations Intergovernmental Panel on Climate Change’s 2007 Summary Report for Policymakers. Eventually, though these gains will be erased.

Juliet Eilperin is The Washington Post's White House bureau chief, covering domestic and foreign policy as well as the culture of 1600 Pennsylvania Avenue. She is the author of two books—one on sharks, and another on Congress, not to be confused with each other—and has worked for the Post since 1998.

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